1. Field of the Invention
The invention relates to the technical field of minimum miscibility pressure determination methods.
2. Description of the Prior Art
When gas is to be injected into an oil-containing reservoir, an important parameter to be taken into account is the minimum miscibility pressure of oil and gas. This pressure, referred to as MMP (Minimum Miscibility Pressure), is the pressure above which gas and oil become miscible. The value of this MMP is fundamental because it determines the recovery ratio of the oil in place: a low miscibility leads to unstable displacements, fingering and therefore poor recovery. In the opposite case, that is a miscible displacement, oil recovery is nearly total in the zones, which are effectively swept. More generally, it is very important to know this parameter for simulation of the gas/oil displacements in reservoirs. Currently, this issue can be of special importance, notably linked with CO2 geologic storage. In fact, the injection conditions and the stored amounts will be greatly influenced by this parameter.
There is therefore a need, particularly in the energy industry, to develop methods allowing fast and accurate measurement of the minimum miscibility pressure (MMP).
In practice, measuring this pressure is long and expensive. A conventional approach carries out several experiments of oil displacement by gas in porous media, at increasing pressures, and in analyzing the results in terms of recovery. In order to obtain the value of this parameter, various procedures are possible. The state of the art can be illustrated by the following document, which compares four known methods of measuring the minimum miscibility pressure (MMP):    Thomas, F. B.; Zhou, X. L.; Bennion, D. B.; Bennion, D. W., “A Comparative-Study of Rba, P-X, Multicontact and Slim Tube Results”, Journal of Canadian Petroleum Technology-33/2 17-26 (0021-9487) (1994).
Pseudoternary Diagrams
It is possible to use pseudoternary diagrams, but determination of these diagrams requires time-consuming experimentations because, on the one hand, equilibria take a long time to obtain and, on the other hand, the results can depend on the path used to reach these equilibria. Furthermore, this method requires arbitrary fractionation of the oil to pseudocomponents, which are not always easy to determine.
Slim Tube Test (STT) Method
A second method sweeps an oil-saturated porous medium with gas, at different increasing pressures. Because of the mobility contrast between the gas phase and the oil phase, the displacement is intrinsically unstable and generates fingering. In order to overcome this difficulty, the injections are carried out at very low flow rates, and porous media of great length and small diameter (hence the designation Slim Tube Test) are used. The test itself carries out displacements at increasing pressures and in studying the oil recovery as a function of the pressure. This recovery increases more or less linearly as a function of the pressure and it eventually reaches a plateau value corresponding to a recovery ratio close to 100%. The value of the minimum pressure for reaching this plateau is the MMP to be determined. This technique is relatively simple but it takes a very long time because of the many displacement experiments that have to be carried out at a low flow rate.
Vanishing Interfacial Tension Method (VIT)
A different approach based on an interfacial tension measurement has recently appeared in the literature. Various authors have started examining this scientific problem. Rao and Takabayashi can be mentioned essentially.    Rao D. N, A New Technique of Vanishing Interfacial Tension for Miscibility Determination, Fluid Phase Equilibria, 1997, vol. 139, pp 311-324.    Takabayashi, K., Ohta, T., Okatsu, K, Interfacial Tension Measurement between Oil and Gas Phase under High Temperature and High Pressure Condition—Effect of Pressure on Interfacial tension and Comparison of Minimum Miscible by Vanishing Interfacial Tension Technique and Slim Tube Test. 25th Annual Workshop & Symposium. Stavanger, Norway, Sep. 5-9, 2004
When two bodies are miscible, the interfaces no longer exist and this property is expressed by a vanishing interfacial tension (IFT). This property is then used to determine the MMP, the interfacial tension is measured as a function of the pressure and the minimum pressure for which the IFT vanishes is thus determined. This technique is definitely faster but it however involves some drawbacks. First of all, measuring the IFT requires knowledge of the density of the two phases under the experimental conditions. It takes a long time and delicate work that requires precise control of the experimental parameters (essentially pressure, temperature and volume ratio between the gas phase and the oil phase). It can furthermore be noted that the closer one gets to miscibility, the longer the time required to reach the thermodynamic equilibria. Finally, and above all, the lower the interfacial tension, the more it is difficult and delicate to measure.
One of the problems associated with measurement by VIT is linked with the equilibrium conditions and the determination of the densities. Close to miscibility, the densities become very close to one another. Now, IFT measurement is directly proportional to the difference between these densities. Therefore, the closer the latter are to one another, the more the error on the density determination leads to a serious error on the IFT determination.